The present invention relates to a magnetic head, in particular relates to such a magnetic head for recording and/or reproducing a magnetic signal on a magnetic medium with high recording density. The present invention relates in particular to a digital signal recording/reproducing system.
The high density magnetic recording technique has been considerably improved, with the recording density becoming ten times as large as that of ten years ago. For instance the recording density up to 8000 bits/mm has been reported in an experiment with a single pole head. However, that value (8000 bits/mm) is obtained merely in an experiment, and the practical value is less than 3000 bits/mm even when a single pole head for vertical recording is used.
Some of the important problems for achieving the high recording density are (1) to improve the remanent magnetization of a medium, (2) to keep the duration between a head and a medium small (less than 1 .mu.m), and/or (3) to improve the sensitivity of a head.
Some of the prior magnetic heads are first described.
(1) A single pole head;
A single pole head as shown in FIG. 1 has the highest recording/reproducing density at present. In FIG. 1, the reference numeral 1.1 is a main magnetic pole, 1.2 is an auxiliary magnetic pole, 1.3 is a coil wound on the auxiliary magnetic pole 1.2, 1.4 is a recording medium which is made of for instance Co-Cr, 1.5 is a base support for supporting said medium 1.4, and 1.6 shows the width of said main magnetic pole 1.1.
In FIG. 1, the leakage flux generated by the recorded signal on the recording medium 1.4 magnetizes the end of the main pole 1.1, then, the leakage flux from the main pole 1.1 is detected by the coil 1.3 wound around the auxiliary magnetic pole 1.2. In this case, the main magnetic pole 1.1 must directly contact with the recording medium 1.4 since the leakage flux from the recorded signal is very weak, and the recording medium 1.4 and the base support 1.5 must be flexible and thin since the duration between the main pole 1.1 and the auxiliary pole 1.2 must be less than 50 or 60 microns for detecting the leakage flux from the small main pole 1.1 (the width 1.6 of which is usually the same as the bit size (0.2-5.0 microns)). Accordingly, a single pole head in FIG. 1 is used only for a floppy disc, but cannot be used for a hard disc which has high recording density since the thickness of a hard disc is larger than 1-2 mm, and a single pole head cannot be used for that thick recording disc.
(2) A magneto-resistance head (MR head);
An MR head is shown in FIG. 2, in which the reference numeral 2.1 is a magneto-resistance element made of for instance permalloy film with the thickness (t), the width (w) and the length (L), 2.2 is a conductor provided at both the ends of said element 2.1. The MR head operates on the principle that the resistance of the element 2.1 depends upon the magnetic flux provided by the recording medium 1.4.
In FIG. 2, when some predetermined current flows through element 2.1, the voltage across the element 2.1 changes according to the magnetic flux recorded on the medium 1.4, and said voltage is the output voltage of the head. The detailed analysis of an MR head is discussed in (IEEE, Trans. on Mag. Vol. MAG-7. No. 1 pp150-154, 1971, USA by R. P. Hunt in Ampex company), and according to that article, the output voltage V is proportional to (1-e.sup.-kw)/kw, where k=2.pi./(.lambda.), and .lambda. is the recording wavelength which is twice as long as the recording bit length. According to said equation, when the wavelength is small, the width (w) must be small in order to obtain the enough output voltage. For instance when =0.2 micron, the width (w) must be less than 1.0 micron, which is unpractical for manufacturing process. The loss increase with the width (w) in an MR head comes from the open magnetic loop of a magnetic circuit.
FIG. 3 shows the improvement of an MR head, and the head of FIG. 3 has the closed magnetic circuit (article MR 82-24 in the Japanese Institute of Electronics and Communication, magnetic recording study group). In FIG. 3, the reference numeral 3.1 is a return path of flux and is made of ferrite, and 3.2 is non-magnetic portion, 2.1 and 2.2 show the same members as those of FIG. 2. The flux signal applied to the end of the MR element 2.1 returns to the recording medium through the return path 3.1. Thus, the reproduction of the signal with the width of 0.13 micron is possible by using the MR element with the width 20 microns on the condition that the relative output level is -45 dB. When the relative output level is -6 dB, said signal width must be 1.27 micron. Further, said output level is obtained on the condition that the medium contacts directly with the head. If the head aparts from the medium by the length L', the output level decreases by e.sup.-kL'. For instance, when the bit period is 0.1 micron and the length between the head and the medium is 0.1 micron, the output level decreases to 0.04, which cannot be reproduced even if that improved MR head in FIG. 3 is used.
Concerning the decrease of the output level by the gap between the head and the medium, the vertical flux component Hy by the vertically recorded signal as shown in FIG. 4 is shown by the following equation. EQU Hy=2.pi.M.sub.r e.sup.-(.pi./d)y (Oe) (1)
where M.sub.r is the remanent magnetization on the medium, d is the bit width, and the thickness loss by the thickness of the medium is neglected on the assumption that the thickness (t) of the medium is considerably larger than the bit width (d). The relations of the equation (1) is shown in the curves of FIG. 5, where M.sub.r =1000 emu/cc.
(3) Optical magnetic reproduction;
FIG. 6 shows the prior optical magnetic reproduction head, in which the reference numeral 6.1 is an optical source by a semiconductor laser, 6.2 is a polarizer, 6.3 is a beam splitter, 6.4 is an analyzer, 6.5 is an optical detector by a photo-diode, and 6.6 is magnetization. The optical beam generated by the optical source 6.1 is converted to a linear polarization by the polarizer 6.2, and the converted linear polarization is applied to the recording medium 1.4. The numeral 1.5 is a base support. The input beam is reflected by the medium, and the polarization direction of the reflected beam rotates on the principle of the magneto-optical effect according to the magnetization on the medium. The reflected beam is applied to the detector 6.5 through the optical analyzer 6.4 (which has the same structure as the polarizer). The strength of the optical beam at the output of the analyzer 6.4 depends upon the direction of the magnetization on the medium, therefore, the output voltage of the optical detector 6.5 depends upon the magnetization on the medium. In an optical magnetic head, the resolving power of the recorded bits is restricted by the diffraction limit. When a semiconductor laser with the wavelength 0.8 micron is used, the diffraction limit of that laser beam is about 0.4 micron. A laser source with the shorter wavelength would be requested for improving the resolving power, however, 0.8 micron wavelength is the limit at present, and no improvement of the recording density is expected so long as the present laser is used.
(4) A copy type optical head (Magnetic recording study group report MR 79-11, Japanese Institute of Electronics and Communications);
FIG. 7 shows a prior copy type optical head, in which 7.1 is a soft magnetic film made of for instance garnet or permalloy, 7.2 is magnetic flux in said soft magnetic film 7.1, 7.3 is leakage flux from the recording medium 1.4, and other numerals show the same memebers as those of the previous figures. In FIG. 7, the soft magnetic film is magnetized by the leakage flux 7.3 from the recording medium 1.4, thus, a magnetic copy of the recording medium is obtained in the soft magnetic film 7.1. The magnetic flux in the film 7.1 is optically read out on the same principle as that of FIG. 6. Although the head of FIG. 7 has the advantage that the medium noise is reduced since recording medium is not directly illuminated, that copy type head of FIG. 7 has still the restriction that the resolving power of the recorded bits depends upon the diffraction limit of the optical beam. Accordingly, the minimum size of the reproducable bit is about 0.5 micron with the use of such a head.
FIG. 8 is a prior modification of a copy type optical head, and the configuration of FIG. 8 is shown in the Japanese patent publication 33781/81, in which 8.1 is a reflection mirror, 8.2 is an optical beam, and other members in FIG. 8 are the same as those of the same numeral members in the previous figures. The feature of the structure of FIG. 8 is that the soft magnetic film 7.1 contacts with the medium 1.4 with some angle P, thus, the reproduction of the shorter wavelength signal is improved. However, as mentioned in accordance with FIG. 5, the magnetic flux at the top of the head is very small when some duration between the head and the recording medium is provided. Further, the optical head has the disadvantage in general that only 1/100 of saturated level of the magnetic change can be used because of the shot-noise of the detector, and thus, the sensitivity of an optical head is small. Further, since the structure of FIG. 8 has no idea to illuminate the area closer than several microns to the end of the soft magnetic film 7.1, the reproduction of a small bit less than 1 micron is impossible.
Another modification of a copy type optical head which is shown in U.S. Pat. No. 3,737,236 is shown in FIG. 9, in which 9.1 is an optical fiber, 9.2 is a core of that optical fiber, and other numerals are the same as those of the same numerals in the previous figures. The soft magnetic film 7.1 in FIG. 9 is positioned at the top of the optical fiber 9.1. Since the diameter of the core 9.2 is less than 50-60 microns, the optical beam can be concentrated on a small area of the soft magnetic film 7.1, and thus, the problem in FIG. 8 is solved by the structure of FIG. 9. However, the head of FIG. 9 has still the disadvantage that no idea is presented for the detection of a signal when leakage flux is weak due to the small recording bit. Further, no idea is presented for compensating the change of the polarization direction in an optical fiber in spite of the fact that an optical head reproduces a signal through the change of the polarization direction of an optical beam.
By the way, the technique for applying bias flux in the magnetization hard axis for improving the sensitivity of the flux detection has been known in "Determination of Low-Intensity Magnetic Fields by Means of Ferromagnetic Film" by F. G. West et al in J. Appl. Phys. 34, pp1163, 1963, and/or "Vapor-Deposited Thin Film Recording Heads" IEEE Trans. on Mag. vol. MAG-7, pp675, 1971. In those prior arts, bias flux is applied in the magnetization hard axis direction, and the flux in a core is detected by a winding wound around the core. Due the presence of the winding, the size of the core must be larger than 500 microns (each side). Therefore, the flux to be detected must be uniform among the wide area which is equal to or larger than the size of the core. Further, due to the large size of the core, a plurality of magnetic domains exist in the core, and the magnetic flux in each domains might be random. Of course, the random flux in each domain decreases the sensitivity of the detection of the flux.
Accordingly, the above two prior arts are impossible to apply for the detector of the magnetic flux which is weak and exists in very narrow limited area, although the sensitivity for detection flux which is uniform in a large area is somewhat improved.
Therefore, the above two prior arts are not suitable for a magnetic head for high recording density in which magnetic flux of each magnetic cell to be detected is limited to a very small area.
As described above in detail, a prior magnetic head has the disadvantage that a small bit (less than 1 micron) can not be reproduced, and therefore, is not capable of reproducing high recording density signal.
Therefore, an improved magnetic head for the use of higher recording density has been desired.